linux-kernel - Re: [PATCH -mm] split_lru: fix pagevec_move

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Date:	Tue, 01 Jul 2008 16:56:25 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	"MinChan Kim" <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, "Rik van Riel" <riel@...hat.com>,
	"Lee Schermerhorn" <Lee.Schermerhorn@...com>
Subject: Re: [PATCH -mm] split_lru: fix pagevec_move_tail() doesn't treat unevictable page

Hi Kim-san,

Thank you for good question.

> > even under writebacking, page can move to unevictable list.
> > so shouldn't pagevec_move_tail() check unevictable?
> >
> Hi, Kosaki-san.
> 
> I can't understand this race situation.
> How the page can move to unevictable list while it is under writeback?
> 
> Could you explain for me ? :)

Actually, I added below assertion and tested on stress workload.
then system crashed after 4H runnings.

----------------------------------------------
static void pagevec_move_tail(struct pagevec *pvec)
{
(snip)
                if (PageLRU(page) && !PageActive(page)) {
                        int lru = page_is_file_cache(page);
                        list_move_tail(&page->lru, &zone->lru[lru].list);
                        BUG_ON(page_lru(page) != lru);  // !!here
                        pgmoved++;
                }
        }
----------------------------------------------------

So, I guess below race exist (but I hope Rik's review)


    CPU1                                       CPU2
==================================================================
1. rotate_reclaimable_page()
2. PageUnevictable(page) return 0
3. local_irq_save()
4. pagevec_move_tail()
                                       SetPageUnevictable()   //mlock?
                                       move to unevictable list
5. spin_lock(&zone->lru_lock);
6. list_move_tail(); (move to inactive list)

then page have PageUnevictable() and is chained inactive lru.
Or, I misunderstand it?


abstraction of related function
------------------------------------------------------------
void  rotate_reclaimable_page(struct page *page)
{
        if (!PageLocked(page) && !PageDirty(page) && !PageActive(page) &&
            !PageUnevictable(page) && PageLRU(page)) {
                local_irq_save(flags);
                pagevec_move_tail(pvec);
                local_irq_restore(flags);
        }
}

pagevec_move_tail(){
	spin_lock(&zone->lru_lock);
	if (PageLRU(page) && !PageActive(page)) {
		list_move_tail(&page->lru, &zone->lru[lru].list);
	}
	spin_unlock(&zone->lru_lock);
}




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